The focus of recent work in our laboratory has been to gain a better understanding
of the forces that are acting within a beating flagellum. Force-calibrated
glass microprobes have been used to measure the stalling force of beating
flagella of reactivated bull sperm. The force to arrest the flagellum averaged
2.52 (± 0.68) × 10-10 N (± SD) in the presence of 1 mM Mg ATP. From this, the
torque acting across the axoneme diameter was calculated. On the basis of the
number of dyneins in the active region of the flagellum pushing against the
probe, we found that each dynein head must contribute approximately 5 pN. As it
is unlikely that a single dynein head could produce much more than this amount
of force, our result suggest that all of the dynein heads must contribute.
Based on this estimate of the force produced by dynein in an arrested
flagellum, we were able to find the t-force in a bull sperm flagellum that is
arrested by shortening. The t-force near the switch-point of the beat is about 0.5 nN/micron, approximately the same as the total dynein
force generated per micron of flagellar length. This value of the
t-force requires that the spokes must bear substantial t-force both before and
after dynein switching. After dynein releases its connections to the
B-subtubule during the beat, spoke #1 should experience approximately 7 pN of
force, pulling it away from the central pair apparatus. This should cause
significant distortion of the axoneme after dynein switching, a concrete
prediction that should be subject to experimental verification.Most recently, we have been observing and
measuring the passive elastic properties of the flagellum after the action of
dynein has been eliminated with sodium metavanadate. We measured the passive
stiffness of the flagellum and observed that the passive flagellum develops a
counter-bend in the distal part of the flagellum, when the proximal portion is
bent with a microprobe. The counter-bend phenomenon is most likely a property
which results from passive elastic linkages between the doublets. We have
observed the counter-bend formation in rat, mouse, bull
and sea urchin sperm flagella.Analyzing
the behavior of the counter-bend, we have obtained estimates of the
interdoublet elasticity, and found that the elasticity is non-linear. At large
shear displacements the elasticity reaches a value of approximately 2.0 x 10-5
N/m per nexin link. This would support the presence of permanent elastic
linkages in the axoneme that can stretch to many times their resting length. Supported by Grant MCB-0110024 from the National Science
Foundation.